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DIURNAL WATER RELATIONS OF WALNUT TREES: IMPLICATIONS FOR REMOTE SENSING
Citation:
Weber, J. AND S. Ustin. DIURNAL WATER RELATIONS OF WALNUT TREES: IMPLICATIONS FOR REMOTE SENSING. U.S. Environmental Protection Agency, Washington, D.C., EPA/600/J-92/032 (NTIS PB92144047).
Description:
Leaf water content (LWC), relative water content (RWC) and water potential (LWP) were measured as indicators of diurnal change in tree water status in an experimental walnut orchard receiving two irrigation treatments: 00% and 33% of potential evapotranspiration (PET). WP had the greatest magnitude of diurnal change throughout the experimental period with minima occurring each day in early to mid-afternoon and maxima between midnight and sunrise. WP was more variable in the 100% PET. WC and RWC were also lower in the afternoon than at night, but had greater variabilitY so the diurnal pattern was not as evident. ince RWC varied by less than 5%, measurements were made for one week only on the 100% PET treatment. omparison between the pattern of LWP and dielectric constants (DC) measured from probes inserted 2 cm into a tree bole showed that both declined nearly in parallel in the morning; however, LWP recovered more rapidly than DC in the afternoon. his temporal discrepancy could be caused by cavitation of xylem elements in the vicinity of the DC probe. icrowave backscatter for L- and X-bands also measured diurnal variation that had local minima in the afternoon, but the pattern among wavelength and polarization signatures was complex. rom these data it appears that radar backscatter can respond to diurnal changes in plant water status. nterpretation of diurnal patterns using a satellite sensor will require models that account for multiple sources of canopy and soil variance, including canopy architecture and soil moisture and more precise understanding of the basis of diurnal patterns in backscatter.